根据官方文档描述，可以使用transaction_read_only参数控制事务的读写状态。但实际测试时发现该参数并不生效。实验结果如下：

[omm@db1 ~]$ gs_guc reload -N all -I all -c "transaction_read_only=on"
Begin to perform gs_guc for all datanodes.

Total instances: 2. Failed instances: 0.
Success to perform gs_guc!

[omm@db1 db1]$ gsql -d postgres -p 26000
gsql ((openGauss 1.0.0 build 0bd0ce80) compiled at 2020-06-30 18:19:27 commit 0 last mr  )
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.

postgres=# select name,setting,boot_val,reset_val from pg_settings where name='transaction_read_only';
         name          | setting | boot_val | reset_val
-----------------------+---------+----------+-----------
 transaction_read_only | off     | off      | off

postgres=# create table t3(id int,name varchar(20));
CREATE TABLE

以上示例发现，虽然使用guc设置transaction_read_only=on，但实际并没有生效，依然可以对数据库做DML/DDL操作。

在实际transaction相关参数查看时发现了default_transaction_read_only参数，该参数默认设置为off状态。

postgres=#  select name,setting,boot_val,reset_val from pg_settings where name like '%transaction_read_only';
             name              | setting | boot_val | reset_val
-------------------------------+---------+----------+-----------
 default_transaction_read_only | off     | off      | off
 transaction_read_only         | off     | off      | off

[omm@db1 ~]$ gs_guc reload -N all -I all -c "default_transaction_read_only=on"
Begin to perform gs_guc for all datanodes.

Total instances: 2. Failed instances: 0.
Success to perform gs_guc!

postgres=#  select name,setting,boot_val,reset_val from pg_settings where name like '%transaction_read_only';
             name              | setting | boot_val | reset_val
-------------------------------+---------+----------+-----------
 default_transaction_read_only | on      | off      | on
 transaction_read_only         | on      | off      | off
(2 rows)

当对default_transaction_read_only 参数做修改后，transaction_read_only参数也随之修改，事务状态也成功变更为read only模式，DML/DDL拒绝执行，示例如下：

postgres=# insert into t1 values(222);
ERROR:  cannot execute INSERT in a read-only transaction

由此可以发现，直接决定事务读写状态的应该是default_transaction_read_only参数，至于default_transaction_read_only 和 transaction_read_only参数的区别和关系，目前官方文档还没有指出，期待进一步完善。